This invention relates to an improved process for making ceramic fibers which contain alumina modified with one or more oxides of zirconium or hafnium.
Alumina fibers have been recognized as useful in high temperature applications for some time. Seufert, U.S. Pat. No. 3,808,015 is an early reference describing ceramic fibers which are predominantly alumina. Subsequently, other workers have obtained improved fiber properties by modifying the alumina with metal oxides such as zirconia or more preferably partially stabilized zirconia, hafnia, or combinations of these oxides. See Wolfe, U.S. Pat. No. 4,753,904 and U.S. patent application Ser. No. 07/100,760.
U.S. Pat. No. 4,753,904 teaches the use of zirconyl acetate as the precursor of choice in the process for making modified alumina fibers. In a typical process described in the '904 patent alumina powder slurry, zirconyl acetate, aluminum chlorohydroxide and an yttrium chloride solution are combined, mixed overnight and concentrated under vacuum with heating. The '904 patent does not report temperatures for the concentration step. The '015 patent reports that the concentration step can be done at the temperature of a team bath (Example 2) or at 20.degree.-25.degree. C. (Example 8). However, several problems, which are not encountered in the production of pure alumina fibers, arise in the process of producing fibers of alumina modified with zirconia and/or hafnia. Spin mix foaming readily occurs when the metal oxide precursor is the acetate salt. Zirconyl acetate is the commonly available zirconia precursor. Foaming, and the subsequent drying of foamed material in the spin mix container, can lead to plugged spinnerets and fiber breaks. Gel formation in the spin mix is another cause of fiber breaks, loss of yield and variability in fiber properties. Gels readily form in mixtures containing a zirconium or hafnium compound in the presence of an aqueous aluminum compound such as aluminum chlorohydroxide.
One attempt to address these problems is described in Japanese Patent Application Publication No. 61-186517. This reference teaches that gel formation in spin mixes for zirconia containing alumina fiber can be minimized by concentrating the aqueous spin mix at 30.degree. C. or less.
Another approach to the problem is described in Japanese Patent Publication No. 59-202239. This reference teaches boiling gelled mixtures to form a sol, but this changes the nature of the alumina-zirconia mixture, and requires addition of an organic polymer prior to spinning.
This invention provides an improved process for making ceramic fibers of alumina modified by other metal oxides derived from their water soluble precursors.